Sunset Glory: Thunderstorms building to the East of Dunnottar Winch Park

General Weather Information

Paragliding (and hang gliding, for that matter) is very weather dependent. Therefore all of us learn more about the weather during the course of our flying careers. This is knowledge that stays with us for the rest of our lives. We never look at the world in the same way, ever again! The wind is not just wind any more, it becomes a thing with direction, strength and related to terms such as flyable, non-flyable, and which site is it good for? Puffy clouds are signals that the thermals are working, and depending on how big they are, whether they are going to over-develop into thunderstorms.

Most of us look at a weather forecast in order to see whether it is flyable or not. For pilots, the most popular programme on TV is the weather forecast! Some of us make a serious attempt to understand the weather and to make a prediction as to what will happen over the next day or weekend. Some regligiously watch the forecasts on the internet. Some simply call the weather bureau to get their forecast. Then there are those (like many farmers and fishermen) who can predict the weather quite accurately through experience and intuition.

For most of us who have neither the inclination nor the time or expertise to interpret the weather data, there are amazingly many weather websites on the internet. What makes looking at them interesting (for me) is that they all make use of the same base data, yet can come to very different conclusions as to what will happen.

Data gathering
Where does the data come from? Forecasts are made from data gathered via weather stations on the surface, and weather balloons and satellite images over the past days from areas as far away as Antartic and Gogh Island, to name a few for South Africa. To get the information of the higher up air masses in a region, weather balloons are sent up at regular intervals. The information retrieved from the balloon is called the sounding data. In South Africa weather balloons used to be released twice every day - at 12 Zulu (other countries send up more) at various centres around the country. (Local time would be 2am or 2pm.) Most forecasts are based on these soundings. But in 5 or 6 hours, much can have changed!

However, from 2011 the Weathe Bureau seems to have made a decision to cut costs even more, by limiting the number of balloons sent up in the country. At present (as in April 2011) it would seem that there are maybe one balloon per day somewhere in the country, not sure where. Unfortunately the lack of sounding data has an impact on the accuracy of the forecasts.

Luckily, these days forecasters also have the added sounding information from departing or arriving aircraft. It is obviously very convenient and important that aircraft cruising the skies can send back information about the weather, temperatures, air pressures and winds that they fly through. Of special interest to us is the information they gather while ascending from or descending to the airports in South Africa. Unluckily though, this information is not always readily available, and definitely not to the layman. The information from these aircraft can be/are incorporated into forecasts, and especially the current daily forecast. Hence local area forecasts (for the day) are sometimes changed by the human forecasters when they get the aircraft soundings. Not all aircraft are fitted with the weather instruments. And even more importantly, these big aircraft do not fly to "small" destinations such as De Aar and other places where sounding data were collected in the "old" days. So the lack of balloon sounding data will be a telling factor.

One can only get the aircraft soundings by asking the forecaster at the aviation met office for it as we plebs are not allowed to get the AMDA data directly from the website. The aircraft soundings are always very revealing as the data shows when there is a strong wind in the upper air (often within a couple of hundred meters above the ground) that could affect our flying on the day. We are fortunately operating within a viable distance from the OR Tambo airport, out at Dunnottar, and even the Vaal Dam. Unfortunately, sometimes the data is not available, although obviously aircraft have arrived or departed. It seems that either the aircraft does not download the data soon enough, or the website is not updated quickly enough after the data was received. Not all aircraft carry the necessary weather instruments, but most international aircraft do.

Computer forecasting models
Nowadays human forecasters do not have to do all the thumbsucking themselves. They have computers that can calculate when what will happen at whichever place based on all the sounding information from all over the show. There are many different forecasting models available. All forecasting models take into account the weather systems that are moving in from the Antartic and elsewhere to create the predictions. It takes only a little change in one aspect of a scientific formula to get a different result. The formula is dependent on the preferences of the programmer or meteorologist who decided on the parameters to be used in the calculation - a real mathematical outcome.

But as said, weather forecasting is a very difficult exercise, seeing that it is an imperfect science, not only in South Africa where there is a relative dearth of data, but world wide - because Nature will not be prescribed to. It will not follow the expectations or guidelines set up by human beings, or for that matter, computers!

Internet forecasts
There are many different forecasts available on the internet. Most pilots prefer a specific forecast website more than the others, normally based on which one is perceived to be the most accurate. At present, for example, Windguru is one of the popular forecast sites. Some aviators, though, prefer the Spotgraph, others favour the Weather Bureau's Aviation forecast. There are more websites in use on a daily basis than can be imagined. In fact, on a day when the weather is supposed to be bad, it will usually be possible to find at least one forecast that predicts good weather!

Elsewhere is a list of websites to look at and use. Every now and again, a new one is found and if we know about it, we will add it to the list. You decide which one is your favourite. :-)

Things to bear in mind
A quick explanation of how the average windspeeds are rated in relation to paragliding:
  • Wind of maximum 12 knots (approx 20kph) on the ground is acceptable for paragliders. For students it should not be more than 8 knots.
  • Wind of approx 15-17 knots (approx 30kph) at 7000' above sea level (approx 700m above ground at Dunnottar Winch Park) is acceptable for paragliders, but can be considered to be too strong for beginner students and marginal for students with a bit more air time.
  • Wind of 20+ knots at any of the higher levels is marginal or not acceptable for paragliders, except for the most experienced pilots.
Note 1: Remember that wind is not static at one speed, but will gust higher or lower.

Note 2: The pilot is always limited with the wind speed he can pull up in. Most pilots, even when experienced, can pull up in max winds of 25kph. There is a small number, but not many, who can pull up in stronger winds. Hang glider pilots can take off and fly in stronger winds, provided they have the experience to handle the glider.

Note 3: Also to be taken into account is the trim speed of the glider. Student paraglider models are usually slower than other gliders, and usually they do not have speedbars connected.

Stronger winds in the upper levels will in general make their way down to the ground. In the process they could lose some of the strength.

Lighter winds higher up will also move downwards. They may indicate that the wind on the ground (if there is any) may calm down or die during the day. Thus lighter winds at higher altitude than stronger winds could mean that the wind may be strong in the morning and less strong in the afternoon.


Such information as the above would make me go out in the afternoon as I would expect the wind to be very strong in the morning, close to the ground if not reaching the ground, but flyable in the afternoon.

Note 4: One knot equates to 1.8 kilometer per hour.

History and the Present
In the old days, we used to do a lot of parawaiting because the weather bureau might put out a good forecast and it ends up howling. Neither they nor us had any information on what was happening in the air layers reasonably close to the ground, at the time that we phoned to get the prediction.

We went to the field or mountain based on the general forecast, and often ended up having coffee in the Wimpy while we hoped that the wind will be shortlived. Wind often tends to die in the later afternoon. At the very least it is smoother which is important for students, low airtime pilots or people who need to build confidence. I have waited on top of mountains for that magical moment oh so often, sometimes having to walk down because it did not happen. When it does, it is worth all the walks down!

The alternative has also happened - the forecast gave bad flying conditions, and we stayed at home in perfect flying weather!

With the availability of all the information these days, it is easier to decide if the weather is flyable or not. This is largely due to our access to the aircraft soundings, and the proximity of Dunnottar Winch Park to the O R Tambo airport. But it is still tricky! We have stayed at home because of a prediction that the strong wind higher up would come down to the ground, and then it did not. Or we go out on the prediction that the strong wind will not come down, and then it does! Strong wind during the day puts people off. These days we often miss the good flying later in the afternoon due to thinking that it will not be flyable at all.

Maybe at present we are too quick to decide that it is unflyable and therefore we just do not go to the flying site. The recession and need to save money has not helped either.

What does all this mean? It means that maybe we should lighten up sometimes (myself included!) and not worry too much if we have an unflyable (middle of the) day at the field. Use it to admire those who have the experience to fly in stronger conditions, and help around those pilots to learn from them. Or just talk to them if they opt to wait as well. The knowledge gained will come in handy. I know it is a toss-up between family and flying, but if you are there, wait for better conditions later in the afternoon - there is usually an 80% chance that conditions will improve after 3pm or 4pm in the afternoon. Or organise your time so that you can come out for the afternoon. You can have some fantastic flights then, even just for a couple of hours. Paragliding is not necessarily a full day exercise.

Interesting clouds @the Dam

Starburst: Interesting clouds at the Vaal Dam, taken on 14 May 2010. Photo by Laura Nelson.

Vaal Dam
From beginning May 2010 my new home has been next to the Vaal Dam. This is a long term dream finally come true - move away from the city to a place where the flying is next door, and I can have more flying and more fun in a beautiful tranquil area, yet still be within easy reach of my target market.

The weather at the Vaal Dam is somewhat different to that of Dunnottar, but being only approx 30-40km further than Dunnottar (for me), I find that I can watch both of the sites and get a fair idea of what to expect. Of course, I am also very fortunate that I have access to a great weather fundi at the Vaal Dam, Ben Mienie. The aircraft info is still valuable, but otherwise we are a little lower than Jhb, as well as further South. I always thought that means more wind, but so far it has not really happened like that. Probably, also, the great mass of water makes for difficulties in predicting accurately for the site. There have been great flying days, as well as days that were flyable (but not great) and days that were totally unflyable. Slowly I will learn to recognise all the signs to read the weather better.

However, who cares too much?! Weather will be weather, however much we try to understand and plan, and life is a gas!

Note 5: If you are interested in interpreting T-Phi diagrams, synoptic charts and many other aspects of the weather information available, read the info on Ulf's Skygod website on the Weather Page.

Author Laura Nelson. Published 17 October 2009.
Updated 8 July 2010.
Updated 16 April 2011.
Photos taken by Ulf Arndt, except where otherwise stated.

Weather info

Interesting lenticular cloud

Capped - Interesting lenticular cloud.

Cloud at Dunnottar

Cumulus clouds

Beautiful flying sky - cumulus clouds. Photograph: First/zefa/Corbis

Strong winds above: A good example of lenticular clouds, taken from the back of Dasklip Pass, Porterville, facing East, in December 2008.

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